In many communication systems, it is required that a power amplifier in a transmission path be able to operate with high efficiency and high linearity simultaneously. Due to the customer desires for different applications and communication standards, such as GSM/EDGE, CDMA2000, WCDMA or WLAN, the power amplifiers of the transmitter path have to deliver a wide range of different output powers. In addition, the different communication standards specify different maximum output powers which the power amplifier in the transmitter path has to deliver. For example the mobile communication standard GSM defines a maximum output power of about 36 dBm, while the maximum output power for a signal according to the WCDMA standard is about 30 dBm. These power levels are given with respect to front end insertion losses.
However, there is a tradeoff between efficiency in high power applications and linearity of the used amplifiers. Normally, a higher linearity in the used amplifier will result in a lower maximum output power and vice versa. As a result, being designed for the highest power level with maximum available efficiency, the power amplifier tends to operate less efficiently at low power levels. This will shorten the lifetime of the battery and reduce talk time duration.
It has been proposed to use different amplifier stages and switch those stages selectively into an amplification path with proper connection to the load. FIG. 4 shows a conventional configurable power amplifier for use in a portable unit. As one can see, the transmitter path comprises three different amplification stages, each of them individually selectable. Three different switches are provided, which connect the output terminals of the respective power amplifiers with the RF output terminal of the transmitter path. Consequently, in such configurable power amplifiers, where each stage has its own output configuration, the described transmitter path provides a variable output power level corresponding to the output powers of each stage and their combinations.
Nevertheless, there is a desire to further improve existing transmitter or amplifier arrangements to reduce power consumption.